Doesn't respond to custom request format

[itsmechlark]

I'm using `wicked_pdf` to handle PDF response but rails use HTML response even I already set route form to "pdf".
The `pdf` mime type already been registered.

At controller:
respond_to :pdf, only: :show

# GET /documents/1
# GET /documents/1.pdf
def show
  retrieve_document_search_hits(@document)
  respond_with(@document) do |format|
    format.pdf { render pdf: @document.display_text }
    format.html
  end
end

rails v4.2.1

responders v2.1.0